Fate Core

The @randsum/games/fate subpath provides mechanics for Fate Core, a tabletop RPG that rolls four Fate dice (4dF), adds a skill rating, and reads the total off the Fate ladder.

Official SiteFate Core by Evil Hat

Installation

bun add @randsum/games

npm install @randsum/games

Usage

import { roll } from '@randsum/games/fate'

// Roll 4dF with no skill modifier
const { result, total } = roll()
console.log(total)  // -4 to +4
console.log(result) // e.g. 'mediocre'

import { roll } from '@randsum/games/fate'

// Add a skill rating (a rung on the Fate ladder, -2 to +5)
roll(3)             // scalar overload: skill of +3
roll({ modifier: 3 }) // object overload: the same roll

import { roll } from '@randsum/games/fate'

const { result } = roll({ modifier: 4 })

switch (result) {
case 'legendary':
case 'epic':
case 'fantastic':
  // A standout success
  break
case 'superb':
case 'great':
case 'good':
  // A solid success
  break
default:
  // Fair and below
  break
}

API

`roll(modifier?: number)` / `roll(input?: { modifier?: number })`

Input:

Parameter	Type	Description
`modifier`	`number` (optional)	Skill rating — a rung on the Fate ladder (`-2` to `+5`), added to the 4dF total

Returns: GameRollResult with:

Property	Type	Description
`result`	`FateRollResult`	The ladder rung the total lands on
`total`	`number`	The 4dF total plus the skill modifier
`rolls`	`RollRecord[]`	Raw dice data from the core roller (each Fate die is `-1`, `0`, or `+1`)

The Fate ladder

Fate dice are six-sided dice with two + faces, two blank faces, and two − faces, read as +1, 0, and −1. Four of them (4dF) sum to a total between -4 and +4. Add your skill rating and read the result off the ladder:

Total	Rung
+8 or more	`legendary`
+7	`epic`
+6	`fantastic`
+5	`superb`
+4	`great`
+3	`good`
+2	`fair`
+1	`average`
0	`mediocre`
-1	`poor`
-2 or less	`terrible`

The open-ended legendary and terrible rungs absorb any total beyond the ends of the ladder.

Error handling

Game roll() can throw two types of errors:

ValidationError (from @randsum/roller) — the skill modifier is out of range (-2 to +5) or not finite
SchemaError (from @randsum/games/fate) — game-specific issues like an unmatched ladder rung

import { roll, SchemaError } from '@randsum/games/fate'
import { ValidationError } from '@randsum/roller'

try {
roll(99)
} catch (error) {
if (error instanceof ValidationError) {
  // Out-of-range skill rating (must be -2 to +5)
  console.log(error.code)    // 'VALIDATION_ERROR'
} else if (error instanceof SchemaError) {
  // Game-specific error
  console.log(error.code)    // 'NO_TABLE_MATCH'
}
}

Types

import type { FateRollResult, GameRollResult, RollRecord } from '@randsum/games/fate'
import { SchemaError } from '@randsum/games/fate'

Schema

This game is powered by a .randsum.json spec that defines the dice pool, the ladder outcomes, and validation rules. The TypeScript code is generated from this spec at build time. See Schema Overview for how game specs work.

Links

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